Flat product of metal, in particular, in the form of a wall for heat exchangers, as well as a heat exchanger with a double layer wall of copper and titanium

ABSTRACT

The invention pertains to a flat product in the form of a wall for a heat exchanger that is operated with cooling mediums in the form of corrosive liquids that contain biological organisms. In order to protect the wall of the heat exchanger against corrosion on the side that comes in contact with the cooling medium and to prevent biological organisms from settling down on the heat exchanger wall, the wall consists of a titanium or titanium alloy layer and a copper/titanium alloy layer that is integrally connected to the titanium or titanium alloy layer by means of an intermediate alloy layer.

BACKGROUND OF THE INVENTION

The invention pertains to a flat product of metal, in particular, in theform of a wall for a heat exchanger which consists of a compositematerial comprising a titanium or titanium alloy layer and a copperalloy layer.

The invention also pertains to a heat exchanger, the wall of which canbe exposed to a cooling medium in the form of a liquid that containsbiological organisms, wherein said wall consists of a copper alloy layerthat faces the cooling medium and a titanium or titanium alloy layerthat tightly adjoins the copper alloy layer and faces away from thecooling medium.

Heat exchangers of this type are known in the form of tubular elements(GB 2 151 744 A). In this known heat exchanger, the tube walls consistof two layers, wherein the inner tube consists of copper or a copperalloy and the outer tube consists of titanium. Both tubes have the samewall thickness of 0.5 mm. The inserted inner tube is hydraulicallyexpanded in order to bring the inner tube in contact with the outer tubeover its entire surface.

The development of such a tubular heat exchanger with double layer wallswas based on the notion that, although titanium provides an excellentprotection against corrosion, biological organisms are deposited orsettle down (biofouling) on its contact surface with cooling mediums inthe form of liquids that contain biological organisms, e.g., seawater orbrackish water. This is caused by the distinctive biocompatibility oftitanium and impairs the heat transmission. In extreme instances, it mayeven occur that the heat exchanger becomes clogged. These disadvantagescan be eliminated if the inside of the titanium tube is lined with acopper tube, namely because copper has a toxic effect on biologicalorganisms. However, it is disadvantageous that the copper or copperalloy only provides a weak protection against corrosion when it comes incontact with aggressive mediums such as seawater (0.5 mm erosion/year),and that the heat transmission from the inner copper tube to the outertitanium tube is also more or less impaired on the boundary between thecopper tube and the titanium tube if both tubes tightly adjoin oneanother (high heat transmission resistance).

In another heat exchanger of this type which is realized in the form ofa multi-tube heat exchanger, the tubes of titanium are connected to atube receptacle plate of a copper alloy (JP 60 11 70 99 A). In case ofan interruption in the operation of the heat exchanger, the watercirculation is shut down such that the protection against electriccorrosion between the different metals of the tubes (titanium) and thetube receptacle plate (copper alloy) is no longer effective. In order toprevent an undesirable electric corrosion in such instances, sacrificialelectrodes that are in intimate and electrically conductive contact withthe titanium tubes and consist of the same copper alloy as the tubereceptacle plate are arranged on the surface of the titanium tubes. Ifthe water circulation is shut down, the occurring electric corrosiononly takes place between the titanium tubes and the sacrificialelectrodes such that the undesirable electric corrosion between thetubes and the tube receptacle plate is prevented. The sacrificialelectrodes are replaced as required such that a permanent protectionagainst electric corrosion between the titanium tubes and the tubereceptacle plate is ensured.

SUMMARY OF THE INVENTION

The invention is based on the objective of developing a flat product ofmetal that, in particular, is suitable for use as a heat exchanger walland protected against corrosion on the side that comes in contact withaggressive liquids that contain biological organisms, in particular,seawater. The flat product of metal is also protected from biologicalorganisms settling down or depositing thereon (biofouling). Theinvention also aims to develop a heat exchanger that is suitable for usewith cooling mediums in the form of aggressive liquids that containbiological organisms. It should, in particular, be possible to achieve apermanently optimal heat transmission without any maintenanceprocedures.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the first objective is attained due to thefact that the copper alloy of the one layer consists of acopper/titanium alloy that is integrally connected to the other layer oftitanium or a titanium alloy, in particular, by means of alloying.

The second objective is attained with a heat exchanger of the initiallydescribed type due to the fact that the two layers are integrallyconnected to one another by means of a copper/titanium boundary layer.This integral connection is, in particular, produced with acopper/titanium alloy layer that preferably extends up to the surfacethat is exposed to the aggressive cooling medium.

In the heat exchanger according to the invention, biological organismsare prevented from settling down or depositing on the heat exchangerbecause the copper or copper alloy layer is exposed to the coolingmedium. When using aggressive, corrosive cooling mediums, a superiorprotection against corrosion is ensured by the titanium that ispreferably alloyed with the copper up to the surface that is exposed tothe cooling medium. In addition, a superior heat transmission betweenthe wall surface that is exposed to the cooling medium and the surfacethat faces away from the cooling medium is ensured due to the integralcopper/titanium boundary layer, in particular, the alloy layer.Consequently, the flat product and a heat exchanger manufactured thereofhave a high degree of efficiency and require no maintenance.

The invention also pertains to a method for manufacturing a heatexchanger according to the invention. In this method, a strip of atitanium material is plated with a strip of a copper material that, inparticular, is thinner than the titanium strip, wherein the thuslyobtained composite strip is shaped into plates or tubes that are joinedsuch that heat exchangers are obtained, and wherein the composite stripis subjected to a heat treatment before or after the plates or tubes areshaped in order to form an alloy layer between the two materials.

In such a method, the integral connection in the form of an alloy layeris formed during the heat treatment. The heat treatment also has asoft-annealing effect, in particular, on the titanium material that washardened due to the plating process. This soft-annealing is particularlyadvantageous if the composite strip needs to be subjected to a moresignificant deformation during the manufacture of the heat exchanger.

If the composite strip still has a sufficient deformability after theplating process despite a certain strain-hardening, it is possible tocut suitable blanks immediately after the plating step and to shapethese blanks into tubes for tubular heat exchangers. In this case, thefinished tubes are subjected to the heat treatment required for formingthe alloy layer.

If the strain-hardening of the composite strip is excessively high afterthe plating step such that the composite strip no longer has asufficient deformability, it is initially required to soft-anneal thecomposite strip, e.g., in a continuous operation, in which the alloylayer is simultaneously formed. Subsequently, the composite strip can beadditionally deformed, e.g., into tubes.

It is particularly advantageous to carry out the soft-annealing and theheat treatment required for forming the alloy layer in one step, i.e.,simultaneously. If it is required to additionally deform the compositestrip after such a double effect heat treatment, it needs to be ensuredthat the copper/titanium alloy layer has a sufficient ductility forparticipating in the deformation without becoming damaged (e.g., due tochipping). This is achieved by choosing the dimensions of the copperstrip so thin in relation to the titanium carrier strip that theformation of an excessively thick and brittle alloy layer is prevented.

In this respect, the invention proposes that the material thickness ofthe layer that faces away from the cooling medium, i.e., the titanium ortitanium alloy layer, amounts to a multiple of the material thickness ofthe layer that faces the cooling medium, i.e., the copper or copperalloy layer. The copper layer preferably has a material thicknessbetween 0.05 and 0.2 mm, and the titanium layer has a material thicknessbetween 0.2 and 2.0 mm. These dimensions ensure that a thorough alloyingof the layer that faces the cooling medium can be achieved.

The heat treatment preferably takes place in an inert gas atmosphere ata temperature between 800° C. and 1000° C. A heat treatment durationbetween 5 and 15 minutes suffices for ensuring the formation of thealloy layer and the soft-annealing effect. The heat treatment of thecomposite strip, from which blanks for the plates and strips for thetubes of the heat exchangers are cut, is preferably realized in the formof a continuous operation.

A composite strip for the plates of a plate heat exchanger or the stripsof a tubular heat exchanger is manufactured by plating a strip oftitanium or a titanium alloy that preferably has a thickness between 0.4and 1.0 mm with a copper foil that preferably has a thickness of 0.1 mm.It is possible to utilize conventional plating techniques, inparticular, the roll-plating technique. The plating ensures amechanically solid bond between the copper foil and the titanium strip.The obtained composite strip is then subjected to a specific heattreatment, in particular, by transporting the strip through an annealingfurnace, in which an inert gas atmosphere is maintained. The treatmenttemperature preferably lies between 800° C. and 1000° C. The strip speedlies between 5 and 20 m/minute. Treatment times between 5 and 15 minutescan be achieved if the path of the strip passing through the furnace isrealized accordingly. This type of heat treatment ensures that thematerial which was significantly strain-hardened during the roll-platingprocess is soft-annealed. In addition, the alloying of copper andtitanium not only results in the formation of an integral boundary layerthat ensures a superior heat transmission between the copper/titaniumlayer and the titanium layer, but also provides an excellent protectionagainst corrosion.

1. A method for manufacturing a heat exchanger including a wall that isexposed to cooling mediums in the form of liquids containing biologicalorganisms, the method comprising: plating a strip of a titanium materialwith a strip of a copper material to form a composite strip; shaping thecomposite strip into plates or tubes; subjecting the composite strip toa heat treatment before or after the plates or tubes are shaped to forman alloy boundary layer between the two materials, the heat treatment ofthe composite strip achieving a thorough alloying of the coppermaterials; and joining the plates or tubes to obtain heat exchangerswith the thorough alloyed copper material of the composite strip facingthe cooling mediums to provide protection against biofouling.
 2. Themethod for manufacturing a heat exchanger according to claim 1, whereinthe heat treatment takes place in an inert gas atmosphere at atemperature between 800° C. and 1000° C.
 3. The method for manufacturinga heat exchanger according to claim 1, wherein the heat treatment iscarried out for a duration between 5 and 15 minutes.
 4. The method formanufacturing a heat exchanger according to claim 1, wherein the heattreatment of the composite strip is carried out in the form of acontinuous operation.
 5. The method for manufacturing a heat exchangeraccording to claim 1, wherein the heat treatment takes place at atemperature which is below the melting points of the titanium materialand the copper material.